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HISTOLOGICAL FEATURES OF SPLEEN

General
Structure of spleen

The
spleen is the largest accumulation of lymphoid tissue in the body. The spleen
is surrounded by a capsule of dense connective tissue from which emerge trabeculae,
which divide the parenchyma, or splenic pulp, into incomplete
compartments. Large trabeculae originate at the hilum, on the medial surface of
the spleen; these trabeculae carry nerves, arteries and veins.

In
humans, the connective tissue of the capsule and trabeculae contains only a few
smooth muscle cells, contrary to what occurs in several animals (eg, horses,
dogs, and cats).

Splenic
Pulp

The
spleen is composed of a network of reticular tissue that contains reticular
cells, many lymphocytes and other blood cells, macrophages, and APCs. The
splenic pulp has two components, the white pulp and the red pulp.
These names derive from the fact that on the surface of a cut through an
unfixed spleen, white spots (lymphoid nodules) are observed within a dark red
tissue that is rich in blood. The white pulp consists of the periarterial
lymphatic sheath (PALS) and the lymphoid nodules, whereas the red
pulp consists of splenic cords(Billroth's cords) and blood sinusoids.

White
Pulp

The
splenic artery divides as it penetrates the hilum, branching into trabecular
arteries of various sizes that follow the course of the connective tissue
trabeculae. When they leave the trabeculae to enter the parenchyma, the
arteries are immediately enveloped by a sheath of T lymphocytes, the periarterial
lymphatic sheath (PALS), which is part of the white pulp. These vessels are
known as central arteries or white pulp arteries. After coursing
through the parenchyma for variable stretches, the PALS receive large
collections of lymphocytes ––mostly B cells––forming lymphoid nodules.
In these nodules the artery, which has now turned into an arteriole, occupies
an eccentric position but is still called the central artery. During its
passage through the white pulp, the artery also divides into numerous radial
branches that supply the surrounding lymphoid tissue.

Surrounding
the lymphoid nodules is a marginal zone consisting of many blood sinuses
and loose lymphoid tissue. A few lymphocytes but many active macrophages can be
found there. The marginal zone contains an abundance of blood antigens and thus
plays a major role in the immunological activities of the spleen.

After
leaving the white pulp, the sheath of lymphocytes slowly thins and the central
artery (arteriole) subdivides to form straight penicillar arterioles
with an outside diameter of approximately 24 μm. Near their termination, some
of the penicillar arterioles are surrounded by a thick sheath of reticular
cells, lymphoid cells, and macrophages. How the blood is delivered to the
trabecular veins is not exactly known and will be discussed later.

The
figure-2 above shows lymphoid nodule of the spleen surrounded by red pulp. A
germinative center (1) and the (eccentric) central artery (2), which is
characteristic of the spleen, are clearly visible.

Red
Pulp

The
red pulp is composed of splenic cords and sinusoids. The splenic
cords contain a network of reticular cells supported by reticular fibers. The
splenic cords contain T and B lymphocytes, macrophages, plasma cells, and many
blood cells (erythrocytes, platelets, and granulocytes).

The
splenic cords are separated by irregularly shaped wide sinusoids. Elongated
endothelial cells line the sinusoids of the spleen with the long axes parallel
to the long axes of the sinusoids. These cells are enveloped in reticular
fibers set primarily in a transverse direction, much like the hoops on a
barrel.

Surrounding
the sinusoid is an incomplete basal lamina. Because the spaces between the
endothelial cells of the splenic sinusoids are 2–3 μm in diameter or smaller,
only flexible cells are able to pass easily from the red pulp cords to the
lumen of the sinusoids. Unfortunately, because the lumen of sinusoids in the
red pulp may be very narrow and the splenic cords are infiltrated with red
blood cells, microscopic observation of a spleen section is not always easy;
observation of PALS may also be difficult.

Closed
and Open Blood Circulation in the Spleen

The
manner in which blood flows from the arterial capillaries of the red pulp to
the interior of the sinusoids has not yet been completely explained. Some
investigators suggest that the capillaries open directly into the sinusoids,
forming a closed circulation in which the blood always remains inside
the vessels. Others maintain that the prolongations of the penicillar arteries
open into the splenic cords, and the blood passes through the space between the
cells to reach the sinusoids (open circulation).

From
the sinusoids, blood proceeds to the red pulp veins that join together and
enter the trabeculae, forming the trabecular veins. The splenic vein
originates from these vessels and emerges from the hilum of the spleen. The
trabecular veins do not have individual muscle walls. They can be considered
channels hollowed out in the trabecular connective tissue and lined by
endothelium.

Functions
of the Spleen

Phagocytosis and Immunological Defense

Because
of its strategic position in the blood circulation, the spleen is able to
filter, phagocytize, and mount immunological responses against blood-borne
antigens. The spleen contains all the components (B and T lymphocytes, APCs,
and phagocytic cells) necessary for this function.

The
white pulp of the spleen is an important production site of lymphocytes, which
then migrate to the red pulp and reach the lumen of the sinusoids, where they
enter the blood circulation. Inert particles are also intensely phagocytized by
spleen macrophages.

However,
it is not a vital organ (not necessary to sustain life).

Also Read:Gross Anatomy of SpleenREFERENCES:Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):